Incandescent light source and condensing system for optical projectors



March 30, 1954 D. l. CRITOPH ET AL 2,673,489 INCANDESCENT LIGHT SOURCE AND CONDENSING SYSTEM FOR OPTICAL PROJECTORS Filed Sept. 21, 1950 RELATIVE INTENSITY 0F ILLUMINATION IO 5 O 5 LATERAL DISPLACEMENT ON TEST SCREEN INVENTORS 9 2 5 DAV/D CUQ/TOPH v.9 QLBIN F). 60/592 Patented Mar. 30, 1954 IN CANDESOENT LIGHT CONDENSING PROJECTORS David I. Critoph, Lancaster,

. Y., assi sar, Snyder, N

SOURCE AND SYSTEM FOR OPTICAL and Albin A. Gradignors to American Optical Company, Southbridge, Mass., a voluntary association of Massach 2 Claims.

This invention relates to picture projectors and the like, and more particularly to an illuminating system therefor employing a filament type those in projectors of earlier construction. the present highly desirable. While incandescent bulbs or lamps of the filaa plurality of closely spaced parallel sections disposed in side-by-side relation in a common plane to provide a concentrated generally rectangular grid-like frontal area of small height and transverse width relative to the optical or projection axis of the instrument. in a projector at the object coils thereof, a relatively efficient light source of fairly high intensity was provided, with the generally substantially completely filled the center-to-edge intensities were not equal, falling off somewhat at the edges, it was nevertheless sufiiciently good to be usable.

However, demands for even greater increases in the light output of such bulbs have, more recently, resulted in the production of projection usetts Application September 21, 1950, Serial No.

bulbs of the bi-plane filament type in which two sets of grids of spaced coils in side-by-side parallel relation laterally offset relative to the coils of the other. When a lamp of this improved type was disposed in the usual manner, greater light output was obtained. its center-to-edge illumination rates were not uniform and were not materially better than that for monoplane filament bulbs, and the presence or absence of the reflector was found to have little effect upon the results obtained.

It has now been found that even higher intensity of illumination and even better or more uniform center-to-edge screen intensities can be obtained in a projector of conventional construction employing the improved bi-plane filament bulb of the type described when following the teaching of the present invention.

It is, accordingly, an object of the present invention to provide in a conventional picture projection system, spotlight or the like a bi-plane filament bulb of the type described and suitable provided, as well as improved uniformity of light intensities from center to edge of said screen.

Other objects and advantages of the present invention will become apparent from the detailed description which follows when taken in conjunction with the accompanying drawing in which:

Fig. l is a schematic showing of an illuminating system for a picture projector or the like which may be readily arranged to embody the present invention;

Fig. 2 is a diagrammatic View of a part of Fig. l but indicating a difierent type light source therefor;

Fig. 3 is a diagrammatic showing similar to Fig. 2 but indicating an arrangement of parts embodying the invention;

Fig. 4 is a diagrammatic showing similar to Fig. 3 but indicating a different arrangement of parts embodying the invention; and

Fig. 5 is a graph showing improved results which may be obtained by a practicing of the present invention.

Referring to the drawing in detail and in particular to Fig. 1, it will be seen that a conventional illuminating system Ill for a picture projector or the like is schematically shown and comprises an objective 12 positioned in front of an apertured film-gate er the like M andin optical alignment therewith along an optical or projeotion axis I is a condenser lens system l6 which in the present instance embodies condensing lenses l8 and 20. The object plane of this condenser lens system is indicated at F and at this plane the filament 22 of a projection bulb of the monoplane filament type would normally be located so that a magnified image thereof will be formed at an image plane F within said objective and of such a predetermined size that the image forming rays entering the objective will substantially completely fill the first lens element thereof. The filament of this projection bulb is of a type having a plurality of spaced coiled sections disposed in side-by-side relation in a single transverse plane so as to provide a frontal area of small generally square or rectangular size. Rearwardly of this bulb 24 and axially aligned with the condenser lens system and the bulb is located a spherical reflector 26 of suitable size and curvature and so located that light traveling rearwardly from the coils of the filament will be reflected and reimaged thereby substantially at the object plane F. In this manner the small rectangular filament area will be effectively substantially completely filled with light, Suitable means, such as a bayonet connection 21, may be provided between the base of the bulb 24 and a socket 28 to insure that the bulb will be properly oriented in the projector and relative to the optical axis i5.

When a bulb of the two-grid or bi-plane filament type, such as is shown at in Fig. 2 having two sets of filament coils 25a and 25b disposed in separate transverse planes one behind the other and in staggered relation to each other, was substituted for the earlier type single grid or monoplane filament type bulb of Fig. 1 and was located with the filament planes at opposite sides of the object plane F, as shown, some increase in illumination occurred. However, the uniformity of intensity across the associated viewing screen was not too good, being only about 70 to 75% at the edge as compared to that at the center. This, however, has been generally considered to be within acceptable limits for projectors partly because it has not been found to be readily possible to obtain better edge-to-center illumination ratios. It is interesting to note also that these results were found to be more or less independent of whether or not the spherical reflector 26 was employed in the usual position behind the bulb.

To better show the non-uniformity of intensities obtained by the bi-plane filament bulb 25 of Fig. 2, a graph is shown in Fig. 5 in which relative intensities of illumination are represented in the vertical direction thereof and amounts of lateral displacement from the center of a viewing or test screen (and from the optical axis) are indicated in the horizontal direction thereon. Upon this graph an intensity curve is shown by a dot-dash line A which indicates the results which were obtained when bulb 25 was operated in the instrument while positioned as indicated by Fig. 2. The illumination in areas near the center of the image field indicated by the vertical lines near the one marked 0 was appreciably higher than that obtained at points materially laterally removed therefrom, such as indicated at 6 or I to either side of the center, and even a greater difference was found to exist nearer the outer edges of the image field as indicated by lines 9 and II).

It has been found that this bi-plane filament type bulb 25 may be rotated in a proper direction about the longitudinal axis of the bulb a suitable angular amount, indicated by X in Fig. 3 (depending upon the size of the individual coils of the bulb employed, the lateral spacing between these coils and the fore and aft spacing between the transverse filament planes of the bulb), and in the bulb construction indicated this amount will be in the neighborhood of 20 to 30 degrees from the transverse object plane F and so that the adjacent relatively staggered coils of the front and rear filament portions will be arranged approximately one behind the other, or nearly so. The direction of rotation preferred is that which will present to the condenser lens system the greater number of exposed individual coiled sections. The result of such an angling of the bulb is that the uniformity of intensity of the light upon the screen will approach much more closely an over-all uniform condition. While the center intensity may be slightly less, the intensity at the outer edges of the illuminated field was materially increased, even in amounts equal to or somewhat greater than that at the center. Such an improved result obtained by proper rotation of the bulb is indie ted by dotted line B in Fig. 5. The direction of rotation of the bulb is important for a rotation of equal amount but in the opposite or wrong direction Will not provide the improved results desired.

Further investigation has also established that additional improved results over those indicated by the curve B in Fig. 5 may be obtained when the bulb 25 is laterally decentered a proper amount and in the correct direction relative to the optical axis i5 of the system (when the coils of the filament are vertically disposed) and while keeping the reflector 26 in optical alignment in the system. Of course, the center of curvature of the reflector should also remain in the object plane F. The longitudinal center line of the rotated bulb is indicated by dotted line 30 in Fig. 4, and the desired small displacement between this line 36 and axis I5 is indicated at 32. The correct direction of this lateral displacement would be the direction in which the bulb is rotated. Or stated differently, if the frontal area of the filament facing the condenser system is to be rotated toward the left to create the desired angle X then the displacement of the bulb should likewise be toward the left. In such an arrangement, the light rays radiated rearwardly from the individual filament coils of the filament will be refiected by the reflector it so as to form images between the filament coils and in such a fashion that much of the light forming these images is allowed to travel on through the grid-like formations and on toward the condenser lens system [6. A plotting of the resulting relative intensities from bulb 25 as were found to exist upon the screen are represented by full line curve C in Fig. 5, and while this curve appears to follow closely the shape of curve B, nevertheless it discloses materially higher intensities at all lateral displacements on the graph.

It is not entirely certain why the improved results just described are obtained by applicants invention. it may be that such a bi-plane filament bulb, so angularly positioned in the projector, will have an illumination pattern of definite shape and will radiate light forwardly and more to one side of the projection axis of the instrument than to the other side while the reflected light rays forming the images of the coils at or near the object plane F may, to a large extent, travel on through the grid formations and through, light from the rear coils passes directly to the condenser system and that some is blocke on. However, it is difficult or impossible to state or determine through the system travels before reaching the first lens element of the condenser. Nevertheless increase in intensity and an improved distribution of the light has been found to occur. It is believed that best results can be obtained when using a bulb of the type shown at when the amount of lateral displacement from the projection axis is made to equal approximately onequarter to approximately one-half the diameter of the individual filament coils. Of course, if the spacing between the coils, or the number of coils, or the spacing between the two planes is relatively greater or smaller as compared to the coil diameters, the best displacement might be a somewhat difierent amount. Actual use of the invention at the present time has indicated that the best results within this small range of adjustwhen this displacement equal to approximately one-quarter of the coil di- As many as six coils in each plane or as few as two, in staggered relation, have been employed to provide frontal filament areas ranging between 6 and 10.5 mm. on a side and have given improved center-to-edge illumination with high intensity when rotated, and with even higher intensities when properly decentered.

Having described first focal plane, the filament of said bulb comprising a plurality of coil-like portions disposed side-by-side substantially parat said second focal plane.

2. In an optical projector combination of a light condensing system having DAVID) I. CRITOPH.

IN A. GRADISAR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,213,974 Taylor Jan. 30, 1917 1,213,975 Taylor Jan. 30, 1917 1,239,371 Evans Sept. 4, 1917 1,313,857 Dennington Aug. 19, 1919 1,709,017 Hill Apr. 16, 1929 1,962,356 Mihalyi June 12, 1934 1,991,861 May Feb. 19, 1935 2,485,377 Grifiin Oct. 18, 1949 

